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Dark matter

For the first time astronomers have mapped dark matter on the largest scale ever observed.

The observations show that dark matter in the Universe is distributed as a network of gigantic dense (white) and empty (dark) regions, where the largest white regions are about the size of several Earth moons on the sky.

AUSTIN, TEXAS-Astronomers have made the largest map yet of dark matter in the universe. This invisible stuff gives off no light, but it does exert gravity on its surroundings. It probably consists of unknown elementary particles, and it’s much more prevalent than the normal matter from which stars, planets, and people are made.

The new map shows that dark matter is concentrated in huge clumps and filaments, with giant, empty regions in between—just as computer simulations had predicted. “We’re very happy to see that our results are similar to what we expected,” says astrophysicist Ludovic Van Waerbeke of the University of British Columbia, Vancouver, in Canada.

Mapping the invisible may sound impossible, but in fact it’s rather simple. Just as an invisible man sleeping in your bed will leave wrinkles in the sheets, the gravity of invisible dark matter produces minute distortions in the observed shapes of background galaxies. Using this “weak lensing” effect to map dark matter is “a first important step to understand the dark Universe,” says Van Waerbeke’s co-worker Catherine Heymans of the University of Edinburgh in the United Kingdom.

Working with the 340-megapixel MegaCam camera on the 3.6-meter Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, the team spent 5 years imaging 10 million galaxies at distances of about 6 billion light-years. “Our map is about a hundred times larger than the largest dark matter map to date,” Van Waerbeke says. A statistical analysis of the shapes of the galaxies revealed the spatial distribution of the intervening dark matter.

The results, presented here at the 219th meeting of the American Astronomical Society, look very much like supercomputer simulations of the evolution of the universe, with dark matter clumped into a “cosmic web” of filaments and knots. The clumpy knots, where most of the dark matter is concentrated, neatly coincide with huge clusters of galaxies, just as cosmological theories suggest.

In fact, says astrophysicist Rachel Mandelbaum of Princeton University, “projects like the CFHT Lensing Survey can be used to test theories of dark matter and general relativity.” So far, Van Waerbeke says, “everything looks okay. The maps show exactly what we expected.” In other words, the results confirm current popular ideas about the physics, makeup, and evolution of the universe.

According to team member Fergus Simpson of the University of Edinburgh, the lensing survey shows not only how dark matter bends light but also how it clumps together over time. The results, he says, already rule out a number of suggested alternatives to Einstein’s general theory of relativity. For instance, Simpson says, a theory known as Modified Newtonian Dynamics is no longer supported by the dark matter data.

Whereas lensing surveys reveal the distribution of dark matter, other types of observations planned for the near future by other projects will shed light on the even more mysterious dark energy that appears to accelerate the expansion of the universe, Heymans explains. A future European space telescope called Euclid will carry out these two types of observations simultaneously, she says. “We really need both.”

The results, presented by Dr Catherine Heymans of the University of Edinburgh, Scotland, and Associate Professor Ludovic Van Waerbeke of the University of British Columbia, Vancouver, Canada, are being presented today to the American Astronomical Society meeting in Austin, Texas. Their findings reveal a Universe comprised of an intricate cosmic web of dark matter and galaxies that spans more than one billion light years.

An international team of researchers lead by Van Waerbeke and Heymans achieved their results by analysing images of about 10 million galaxies in four different regions of the sky. They studied the distortion of the light emitted from these galaxies, which is bent as it passes massive clumps of dark matter during its journey to Earth. Their project, known as the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), uses data from the Canada-France-Hawaii Telescope Legacy Survey. This accumulated images over five years using the wide field imaging camera MegaCam, a 1 degree by 1 degree field-of-view 340 Megapixel camera on the CFHTin Hawaii.

Galaxies included in the survey are typically six billion light years away. The light captured by the telescope images used in the study was emitted when the Universe was six billion years old – approximately half the age it is today.

The team’s result has been suspected for a long time from studies based on computer simulations, but was difficult to verify owing to the invisible nature of dark matter. This is the first direct glimpse at dark matter on large scales showing the cosmic web in all directions.

Professor Ludovic Van Waerbeke, from the University of British Columbia, said: “It is fascinating to be able to ‘see’ the dark matter using space-time distortion. It gives us privileged access to this mysterious mass in the Universe which cannot be observed otherwise. Knowing how dark matter is distributed is the very first step towards understanding its nature and how it fits within our current knowledge of physics.”

Dr Catherine Heymans, a Lecturer in the University of Edinburgh’s School of Physics and Astronomy, said: “By analysing light from the distant Universe, we can learn about what it has travelled through on its journey to reach us. We hope that by mapping more dark matter than has been studied before, we are a step closer to understanding this material and its relationship with the galaxies in our Universe.”

For Dr Christian Veillet, CFHT Executive Director, this dark matter study illustrates the strong legacy value of the CFTHLS: it is now enabling exciting results obtained by teams from many nations which use the CFHTLS images retrieved from the Canadian Astronomy Data Centre where they are archived and publicly available.

Professor Lance Miller, from Oxford University said: “This result has been achieved through advances in our analysis techniques which we are now applying to data from the Very Large Telescope’s (VLT) Survey Telescope in Chile.”

Professor Koen Kuijken, from Leiden University, said: “Over the next three years we will image more than 10 times the area mapped by CFHTLenS, bringing us ever closer to our goal of understanding the mysterious dark side of the Universe.”